Rejection of emails - &quot;Possible Abuse&quot;

I am the Coordinator of our local Neighbourhood Watch and regularly send emails to its 230 members alerting them of suspicious incidents in our neighbourhood - mostly at the request of our local Constabulary. In addition I send residents of the village I live in emails notifying them of village events and other matters of interest to them which they submit to me for publication.
About 18 months ago the emails that I was sending were constantly being rejected by Yahoo with the following reason " 554 Transaction failed : Cannot send message due to possible abuse; please visit http://postmaster.yahoo.com/abuse_smtp.html for more information" which suggested that I should send them in batches of no more than 100 at a time. I told Yahoo that the work I performed was voluntary and often took up a lot of my time and I therefore considered their request to send circulars in batches of 100 unreasonable and inconvenient and asked them if they would allow me to send emails to all Neighbourhood Watch members in one batch. As I did not receive a reply I had no option but to follow their instructions but am now receiving the "possible abuse" messages again and the rejection of the emails even when I send them in batches of only 75 emails!
Can anybody suggest what I can do to avoid the rejection of the emails I send? I have asked Yahoo and BT (my service provider) and get nowhere.